Voltage controlled oscillators (VCOs) are well known and widely used in the electronics industry. Within the digital communications field, VCOs are used in a variety of applications. Such applications include, for example, frequency synthesizers, signal generation, (e.g., serial transmission clock recovery) and the like. VCOs are typically designed to perform within a given set of boundary conditions and to perform according to a specified standard. Typical conditions include, for example, performance over operating temperature ranges, sensitivity to vibration, output sensitivity to interference, and the like. Typical performance standards include, for example, output signal frequency stability, output signal programmability, and the like.
A typical prior art VCO circuit generates an oscillating output signal having a specified frequency. The signal can have several different wave forms (e.g., square, saw tooth, triangular, etc.). The frequency of the output is tunable and is a function of an input voltage, an external resistance or capacitance, or the like. The type of application in which the VCO circuit is used dictates its operating conditions and performance requirements.
In addition, the type of application also largely determines type of fabrication technology used to manufacture the VCO. A large number of modern digital integrated circuits are fabricated using well known and widely used CMOS technology. Where the VCO circuit is included in a CMOS IC (integrated circuit), it is usually fabricated in CMOS (e.g., fabricated using CMOS process technology).
There is a problem, however, when the application in which the overall IC is used requires the VCO circuit to perform at very high operating frequencies. For example, where the IC is part of a high speed serial transmission system (e.g., gigabit ethernet) it is important that the output frequency of the VCO circuit be stable and be a consistent function of the control inputs (e.g., voltage, capacitance, and the like) while the output frequency is 1 GHz or greater.
In a case where a prior art VCO circuit is used in an application for clock recovery in a gigabit serial transmission system, it is important that the output frequency remain stable and the output waveform remain within specified limits, even at the output frequencies of 1 GHz or more . The output frequency is used to reconstruct a serial transmission clock signal, which in turn, is used to sample data on a serial transmission line. Distortion, defects, irregularity, or variation in the VCO output frequency or the waveform can have a very detrimental effect on the reconstructed clock signal, and hence, could lead to sampling errors, lost data, decreased throughput, or other such problems.
Consequently, for these very high frequency applications it is important that the VCO circuit provide a very stable, "glitch" free output signal at the specified frequency (e.g., 1 GHz). However, prior art CMOS VCOs cannot reliably function at such high frequencies. Prior art CMOS VCOs cannot reliably generate output signals having an acceptable waveform (e.g., free of glitches) and having acceptable stability. Accordingly, system designers are forced to use other, less desirable, alternatives (e.g., transferring data on both the rising and falling edges of a lower speed clock signal, using a separate, non-integrated, non-CMOS VCO, etc.) for high speed applications, such as clock recovery in gigabit serial transmission applications.
Thus, what is required is a CMOS VCO circuit which solves the high speed operation problems of the prior art. What is required is a circuit capable of reliable operation at frequencies of 1 GHz and above. What is required is a circuit which produces a stable output signal having a frequency of 1 GHz or greater and having a waveform free of defects and irregularities. The present invention provides an advantageous solution to the above requirements.